The performance of spraying systems, as measured by the droplet size spectra and distribution pattern of the spray is highly dependent on the fluid properties of the liquid being sprayed. The classic fluid properties such as density, equilibrium surface tension, dynamic surface tension, shear viscosity, extensional viscosity, void fraction of incorporated gasses, etc., all affect the behavior of the liquid as it passes through an atomizer, and subsequently, the characteristics of the resulting spray. When sprays are produced for coating, drying and other processes, the spray characteristics are critical factors in the performance of the process and using the spray and the resulting quality of the product.
To achieve desired spray characteristics, the proper nozzle or atomizer must be selected and the optimal operating conditions of the atomizer and fluid handling system must be determined for the fluid to be atomized. Selection of the nozzle and determination of the operating conditions can be an extensive, iterative, experimental process due to the complexity of the fluid-atomizer interaction. Especially for complex fluids that are heterogeneous, non-Newtonian or otherwise difficult to characterize, a priori predictions of sprayer performance can be difficult and inaccurate. Subsequent changes in the fluid composition, wear in the atomizer or other departures from the original test conditions can require repeat experiments.
Laboratory measurements of fluid properties can be tedious, expensive and time consuming. Additionally, the measurements are often made using standardized techniques that do not closely approximate the conditions in the actual spraying process. These conditions can include turbulence in the flow system, shear rates during flow and atomization, spatial and temporal gradient in temperature, reactions in the fluid, etc.
Likewise, the measurement of spray characteristics such as droplet size spectra, spatial distributions and patterns and droplet velocities requires specialized, expensive equipment and technical expertise in proper sampling in data interpretation. With limited feedback on atomizer performance, especially in processes where the sprays or products are not visible to system operators, generation of poor quality sprays with undesirable characteristics is often undetected until adverse consequences have occurred.
While these challenges are present for any spraying applications, a particular problem exists for agricultural spraying where the spray fluids can be mixtures of pesticides, fertilizers, surfactants, shear-inhibitors, buffers, adhesives and other supplemental agents known as spray adjuvants. These mixtures are highly variable and often created for specific fields to be treated; the physical properties of these mixtures are very complex and it is difficult to predict how the fluid mixtures will behave in a given spray system.
Spray drift, or the inadvertent movement of small spray droplets from the target site to a non-target area, is a significant issue presently facing agricultural applicators throughout the United States. The strongly related issues of spray quality, that is, coverage of the target and efficacy of the product against the target pests are also of great concern. Agricultural applicators desire to use the best drift management methods and equipment to provide the safest and most efficient applications of pest control materials to the targeted pest. They are responsible for making good decisions in the field on a daily basis. Spray droplets that drift off-site or are not correctly applied to the target crop or pest represent wasted time, resources and result in environmental pollution. This results in increased costs for the crop grower and, subsequently, to the consumer. In addition, materials such as herbicides and defoliants that drift off-site can result in a serious financial liability if surrounding crops are damaged.
The minimization of off-site movement of agricultural sprays is to the benefit of all concerned—applicators, farmers, regulators, the public and the environment. Applicators need additional methods and equipment to balance or optimize spray tank adjuvant performance and economics to achieve drift mitigation goals for a given application. In particular, a need currently exists for an apparatus and method for assisting applicators in determining the best possible application parameters to help meet product instructional label criteria and mitigate spray drift. It has long been understood that spray droplet size is the most important variable in spray coverage, performance and spray drift control or mitigation. For an agricultural spray dispensed from an aircraft, spray nozzle selection is the first factor considered when attempting to influence the spray droplet spectrum. Second are the operational factors that influence atomization. These include nozzle angle or deflection to the airstream, aircraft speed, and spray liquid pressure. Spray tank additives or adjuvants play an auxiliary role in spray droplet spectra. There are currently over 416 adjuvants marketed in California alone according to Crop Data Management Systems (Marysville, Calif.). Adjuvants are classified as surfactants, spreaders, stickers, deposition aids, activators, humectants, antifoamers, wetting agent, and drift reduction agents. These agents are added to the spray tank mix that may include a number of active ingredients in the pesticide formulations. Adjuvants can aid in the product making better contact with the pest by spreading it over the leaf surface or the body of the insect pest. Adjuvants can also reduce the likelihood of the product dripping off the leaf onto the ground. Similarly, excessive or incorrect adjuvant use can cause the product to drip or run off the leaf. Adjuvants also can be very useful in helping the product “stick” to the leaf or crop, preventing runoff during rain or irrigation. Finally, adjuvants are often marketed as drift reduction agents. The addition of an appropriate adjuvant can tend to increase droplet size, which generally reduces driftable fines. Unfortunately for applicators, sometimes recommended mixtures are found to be “poor combinations”, even if applied under “ideal climatic conditions”, when damage to crops, crop losses and drift problems are experienced.
Droplet size is determined by the physical properties of the components of the droplet fluid—in this case, the tank mix, usually composed of water, pesticide active ingredient formulations and adjuvant(s). The key properties of the tank mix that have a significant effect on droplet size and the resulting atomization profile are: dynamic and equilibrium surface tension, extensional viscosity, and shear viscosity. Each time the applicator adds something to the tank mix, the physical properties of that tank mix change and that changes the atomization profile. Because of the continued development and advancements in adjuvants, a need also exists for a system and method for assisting applicators in making sound decisions about the addition of these products and the subsequent impact their addition will have on the actual application, both for spray quality and for drift potential.
What is needed by all applicators, not just aerial but also for field crop boom applicators and orchard and vineyard air carrier applicators, is a field method to estimate the atomization characteristics of particular spray mixes that they are about to apply, especially if the mix is used only occasionally. By knowing the atomization characteristics of the mix, one can then choose the proper nozzle and spray conditions to avoid drift and optimize deposit and efficacy. One may even, upon getting the information, decide to delay an application until better environmental conditions exist.
In a broader sense beyond pesticide spraying, optimizing any spraying system requires that the atomizing properties of the fluid be known. The complexity of fluid properties and the complexity of the fluid-nozzle interaction make the prediction of the atomizing properties from laboratory measurements of individually-measured fluid properties (e.g., dynamic and equilibrium surface tension, shear viscosity, extensional viscosity, density, etc.) difficult and inaccurate. The difficulty of selecting and conducting the most appropriate laboratory tests of the fluid properties, combined with the uncertainty of prediction models of droplet size spectra from the resulting measurements, lead to the need for a more direct and simple method for the end user to determine atomization characteristics of a fluid before undertaking a spray operation.